1. Field of the Invention
The present invention pertains to explosive devices such as blasting caps. More particularly, the present invention is related to techniques for initiating the detonation of an explosive train by electroexplosive devices, and finds particular application in the field of jet guns used to perforate casing in walls such as oil and gas wells.
2. Description of Prior Art
It is well known to line wells, particularly oil and gas wells, with casing cemented in place to prevent the unwanted intrusion of fluids and debris into the well. In a typical operation, a well is cased in sections as the well is being drilled to the desired depth. Such as well may traverse one or more formations containing fluid which is sought to be produced by the well. However, once the well is lined with cemented casing, the desirable formations as well as the remainder of the underground structure are sealed off from the well. A tool containing one or more perforating guns is lowered by a cable to position a perforating gun at the level of a formation from which fluid is to be produced. The perforating gun is then fired by an electrical signal controlled at the surface. In practice, more than one such gun may be fired in response to the same electrical signal to produce multiple perforations in the casing at the same formation. Also, with a multiple gun tool, the individual guns may be selectively fired to produce perforations at various levels in the well as the tool is selectively positioned at the various formations.
Within the jet perforating gun, the shaped charge which provides the jet which produces the actual perforation is at the end of an explosive train which begins with a blasting cap in the form of an electroexplosive fuse. Firing of the fuse by an electrical signal detonates a booster which in turn detonates the explosive material of the shaped charge.
Current United States Government Regulations prohibit the transport of charged perforating guns over highways with the blasting caps installed in the guns. Consequently, it is the current general practice to only partially assemble the perforating gun prior to transport of same to the well site. There, the explosive train is made up and the perforating gun is completely assembled for use in the well. However, to make up the explosive train at the well site, it is necessary to handle the explosive devices themselves as well as to complete the electrical connections between the blasting cap and the electrical system which provides the firing signal. This is an inherently dangerous operation and may be required to be performed under less than ideal conditions, particularly where the well site is exposed to extreme weather or other deleterious conditions.
U.S. Pat. No. 148,338 issued to Varney discloses an early type electroexplosive blasting cap. U.S. Pat. No. 4,011,815 issued to Garcia discloses an explosive train arrangement within a perforating gun whereby an electrically-actuated detonator may be moved between two positions. In one such position, the detonator is sufficiently close to a receptor explosive wherein the explosive train is armed. Thus, in such configuration, firing of the detonator would cause detonation of the receptor explosive as well. In the other position, the detonator is sufficiently removed from the receptor explosive to prevent detonation of the latter upon firing of the detonator. An external indicator is provided whereby an operator may visually determine whether a detonator within the perforating gun is in the unarmed configuration. The latter patent also discloses an arrangement whereby a barrier may be selectively positioned between the detonator and the receptor explosive, the presence or absence of the barrier again being indicated externally for observation by an operator.
Inspite of the placing of the apparatus described in the '815 patent in a "disarmed" configuration, it appears that the transport of such an otherwise completely assembled perforating gun on the public highways is still precluded by current government regulations.
It is thus desirable to provide an explosive train assembly wherein an otherwise assembled perforating gun may be considered disarmed and in condition for transport on the highways in compliance with government regulations, while at the same time not requiring extensive assembly at the well site.